The resilience of a socio-ecosystem is usually tested by its ability to persist and maintain its functionality while undergoing change due to disturbances. However, what happens when the disturbances are too rapid, too detrimental, and too overpowering for a socio-ecosystem to maintain its functionality?
Water hyacinth invasion has been such a disturbance. It has been causing tremendous destruction for many socio-ecosystems around the world (Figure 2). Studies indicate that the alien plant has an alarmingly rapid growth rate; it is estimated that 10 plants can reproduce 655,360 plants in just 8 months under advantageous conditions (Datta et al. 2021). In addition, the United Nations Environment Programme (UNEP 2013) suggests that water hyacinth infestation in water bodies cannot be easily managed and controlled; commonly used strategies to control the weed have proved to be unsustainable in many instances, resulting in the menace weed re-appearing over time.
Therefore, there is a significant need for more effective management strategies to successfully restore and rehabilitate aquatic ecosystems that have been colonised by water hyacinth and to ensure that the weeds’ detrimental effects are alleviated.
Detrimental effects of water hyacinth on socio-ecosystems
The invasive plant has been categorised as one of the worst weeds in the world (UNEP 2013) because of its detrimental effects on the functionality and productivity of aquatic ecosystems, livelihoods and the economy. Honlah et al (2019) indicates that in most tropical regions the plant has no predators, allowing it to rapidly infest aquatic systems in these regions, thus overpowering existing indigenous plant and animal species. Eutrophication and high temperatures are other factors that can exacerbate the spread of the weed in water bodies (UNEP 2013).
The plant is characterised by large leaves, which prevent sunlight from penetrating aquatic bodies leading to reduction of photosynthesis and affecting the growth of plant species (Thamaga and Dube 2019). The large leaves also disturb the oxygen transfer from the air to the water causing lower oxygen concentrations (UNEP 2013). Oxygen depletion is further accelerated by the death and decay of the weed, contributing to deaths of fish in some instances, and an increase in eutrophication which further propels the spread of water hyacinth. The depletion in oxygen concentrations reduces water quality and quantity (UNEP 2013), making the water unsafe for drinking and farming purposes. The interrupted composition of water bodies does not only affect the functionality and productivity of the aquatic ecosystem, but the whole socio-ecosystem becomes affected.
The deaths of fish due to low oxygen concentration in the water severely affects the livelihoods of community members who make a living through fishing. In addition, the water hyacinth poses a threat to social health and well-being, with the plants’ mats harbouring dangerous pathogens and vectors such as mosquitoes and snails, which are responsible for promoting diseases such as malaria, cholera, dengue, schistosomiasis, and chikungunya (Datta et al. 2021; Honlah et al. 2019). For example, in Lake Victoria a sharp increase in cholera cases was observed in a riparian community after the weed infested the Lake (UNEP 2013).
The reduced water quality and quantity due to the presence of water hyacinth in aquatic ecosystems has a negative effect on farming practises, more so on small holder farmers. In Ghana 81.8% of farmers in riparian communities who have to cross water bodies to reach their farms indicated that heavy water hyacinth infestation made it difficult to commute to their farms, thus causing their farms to be neglected, leading to their farm produce being destroyed (Honlah et al. 2019). Clogged waterways also reduce fishing practises as fishermen cannot navigate water bodies in order to catch fish. In Kenya, fish catches reduced by 45% due to the presence of water hyacinth (UNEP 2013). This has severe effects on the economy, particularly in vulnerable communities in developing countries.
Applying space-based technologies to detect and monitor the growth of water hyacinth
The application of remote sensing to monitor and manage water hyacinth has had much recognition over the past years with a substantial amount of literature focused on this cause (Thamaga and Dube 2019. This is because satellite images present vast spatial and temporal data, allowing for early detection of new growths and identification of water hyacinth in different geographical locations around the world. Space based data is beneficial in assisting professionals to determine whether their management strategies to combat the spread of the weed are effective or not (Thamaga and Dube 2019). The application of most space-based data is inexpensive and a timely processing can take place, which eases the work of professionals. Sensors with high spatial resolution are of great benefit in identifying the weed in water systems. Examples include but are not limited to Sentinel-2, Landsat-8, Medium Resolution Imaging Spectrometer (MERIS) and the Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER). However, according to Datta et al. (2021), imagery from these sensors may be affected by weather conditions such as cloud cover, therefore needing to be coupled with other sensors which are not affected by metrological conditions, such as Sentinel-1 and NASA-ISRO Synthetic Aperture Radar (NISAR). The sensors capture the weed by its reflectance, and some hyperspectral sensors are able to discriminate the weed from other vegetation through its leaf optical properties, biochemical and biophysical properties (Datta et al. 2021).
Table 1 (see below) indicates some of the commonly used sensors for mapping water hyacinth, and their spatial, spectral and temporal resolution, as well as their appropriateness.
Imagery from sensors is analysed using machine learning algorithms, which assist in accurately discriminating and classifying water hyacinth from other vegetation in ecosystems (Thamaga and Dube 2019), therefore providing more detailed and reliable results. Commonly used methods of classification include tools such as maximum likelihood classification, the use of Normalised Difference Vegetation Index (NDVI) indices for vegetation classification (Datta et al. 2021) and decision tree approaches which help improve visibility of land cover in low resolution images with a water hyacinth classification accuracy of 61.9% (Thamaga and Dube 2019).
Furthermore, results obtained from satellite images can be used collaboratively with modern technologies, such as ground sensors and aerial surveys in order to gain better insight into water hyacinth infestation location and extent (Datta et al. 2021). Using modern instruments such as spectro-radiometers and drones to better understand satellite imagery is key to more effective identification, control, and management of the invasive weed (Datta et al. 2021). Smartphones can also be useful in getting citizens involved in reporting the water hyacinth infestations, making it easier to detect the weed (Datta et al 2021).
Successful applications of space-based technologies
The application of space-based technologies has resulted in a better understanding of water hyacinth in different water bodies around the world. It has thus rendered a chance for effective decision making in management strategies and control of water hyacinth. For example, a study by Dube et al. (2017) appreciates the latest Landsat-8 Operational Land Imager (OLI) sensor as it demonstrated great accuracy (72%) in capturing water hyacinth in Lake Chivero in Zimbabwe. By using the sensor, professionals were also able to define and locate new, intermediate, and old growths of the weed in Lake Chivero. Identification based on satellite imagery assists professionals in better understanding the behaviour of the plant. The possibility to observe temporal and spatial distribution of new growths may also assist in determining whether the cause of new infestation is seasonal or attributed to other factors such as eutrophication and temperature.
A study of water hyacinth invasion in Lake Tana, Ethiopia, indicates that the use of the European Space Agency’s PlanetScope imagery together with classifiers to detect water hyacinth in the lake was very accurate and beneficial. Furthermore, information such as rainfall data, temperature and water levels of the lake was also determined to further investigate the possible cause to water hyacinth infestation in the lake. The study suggests that water hyacinth infestation occurred during the rainy season. It was noted that runoff carried pollution and sediments with phosphorus into the lake during this period, which was responsible for the growth of the hyacinth. This valuable information was used to plan management and mitigation practises to curb the spread of the weed and to eliminate it (Worqlul et al. 2020).
Lastly, Datta et al (2021) highlights the successful and beneficial use of space-based technology together with other technologies by demonstrating the use of a spectro-radiometer in lakes in Germany, whereby the reflectance spectra of vegetation and sediments was used to control the water column and the atmosphere deviations in remote sensing data. This ultimately allowed better interpretation of the satellite imagery and more accurate mapping and detection of the weed. It can be concluded that the coupled use of remote sensing with other technologies has great potential and should be explored and implemented further.
Implementation of space-based technologies for restoration and rehabilitation of infested aquatic ecosystems
Using satellite data has proved to be beneficial in detecting and better understanding the spatial and temporal distribution of the plant, as well as identifying possible causes of infestation. Obtaining this information is vital in reviewing current strategies for mitigation and control of infestation (Thamaga and Dube 2019). Control and removal of the weed helps the system restore its natural functionality and eventually the rehabilitation of socio-ecologies. Although systems may not always go back to their original form, effective and timely rehabilitation methods prevent extensive damage to the system. As mentioned, remote sensing has the ability to assist with early detection, which helps with faster action towards restoration of the aquatic ecosystem. Scientists have increased opportunities to better manage and control ecosystems, with remote sensing data being freely accessible to a wide audience around the world. Additionally, the detection of the weed and appropriate management of water hyacinth can also be beneficial for economic purposes, as the plant can be used for animal feed, arts and crafts, biogas production, etc. (UNEP 2013).Conclusion
The notorious water hyacinth has given many socio-ecosystems around the world a rude awakening, derailing order and functionality and threatening human health and livelihoods. However, opportunities afforded by space-based technologies to stakeholders have been of great significance and have assisted in understanding potential cause of infestation, spatial and temporal growth of the weed, as well as early detection of the weed. Better decisions can now be taken on how to appropriately manage the spread of the weed, thus resulting in many aquatic ecosystems around the world being restored and rehabilitated accordingly.